Submicron lignin dispersions

ABSTRACT

A lignin dispersion of submicron particles with film forming, binding, and adhesive properties in which the median particle diameter of the lignin in dispersion preferably is less than about 300 angstroms. The aqueous dispersion is formed by reacting about 12 to 18% lignin by weight in aqueous solution with from about 2 to 15 moles ammonia, 4 to 12 moles formaldehyde, and 0.25 to 1.25 moles sodium hydroxide or potassium hydroxide at a temperature of at least about 150° F. for a period of time to produce a colloidal dispersion of lignin particles.

This invention relates to small particle size lignin materials and to amethod of producing lignin products having a submicron median particlediameter in colloidal dispersion, which products can form a film and aresuitable for use as chemical additives in other chemical processes andcompositions. The submicron lignin dispersion is characterized by havingmonodispersed particles, necessary for its film forming properties.

BACKGROUND OF THE INVENTION

Lignin is a complex, high-molecular weight polymer occurring naturallyin close association with cellulose in plants and trees. In thepaper-making industry, lignin may be recovered as a by-product of thecellulose product by two principal wood-pulping processes known as thesulfite process and the kraft process. In the sulfite pulping process,lignin is solubilized from the cellulosic portion of the wood pulp bydirect sulfonation, while the kraft process is based on an alkalinedegradation mechanism causing cleavage of β-aryl ether linkages in thepolymeric lignin which subsequently result in chemical functions of thephenolic and carboxylic type. Kraft process lignin generally is isolatedby acid precipitation from the black liquor of a kraft pulping processat a pH below the pKa of the phenolic groups.

Depending on conditions under which the lignin is precipitated, theprecipitated lignin may be either in the form of free acid lignin or alignin salt. If lignin is precipitated at a high pH, such as about 9.5to 10, the lignin is obtained in the form of a salt. If this lignin isfurther processed by washing, acidifying to a low pH, such as about 2 to5, and further washed so as to be substantially free of salt andash-forming ingredients, free acid lignin, known as "A" lignin, isobtained. A monovalent salt of lignin, such as an alkali metal salt oran ammonium salt, is soluble in water, whereas free acid lignin andpolyvalent metal salts of lignin are insoluble in water.

The high degree of chemical activity which is characteristic of ligninpermits the preparation of many organic derivatives. Lignin by-productshave been variously employed in other chemical compositions assurfactants, extenders, dispersants, reinforcers, absorbents, binders,sequestering agents, emulsifiers and emulsion stabilizers, and as astabilizing and protective colloid. Lignosulfonate compounds,particularly sodium salts of lignosulfonates, have been employed asadditives and dispersants in textile dyestuffs and printing pigments.Sodium salt sulfonated lignin by-products have been sold for many yearsunder the trademark Indulin® by Westvaco Corporation of NorthCharleston, S.C.

Certain lignins have been utilized as a binding agent for water-basedink systems incorporating pigments and/or dispersed dyes. U.S. Pat. No.2,525,433 discloses the use of lignin dissolved in a water-missiblesolvent as a binding material for a pigment in printing ink. The methodof U.S. Pat. No. 2,525,433 requires the use of a solubilizing agent,generally an organic amine or alcohol, to completely dissolve the ligninparticles. The resultant solution when used with pigments for inkapplication is limited to newsprint or other porous paper. There is nofilm-forming action of the lignin and no solid particles remain afterthe lignin is solubilized. Adhesion to coated paper, calendered paperand sized papers is poor with excessive rub-off resulting.

Lignin is recovered from the black liquor of a kraft pulping processe bydecreasing the liquor pH by acidification with carbon dioxide, organicor inorganic acids or bivalent metal salts. When the liquor pH isreduced from its normal pH of 13.0-13.5, the lignin precipitates as thesodium salt in the form of agglomerates of around 70 microns mediandiameter (700,000 angstroms (A)) or larger. The lignin-black liquorslurry is filtered to remove the liquor and the lignin particles arerecovered as a filter cake. This cake is washed to remove the occludedblack liquor and the purified lignin solids are acidified to pH 2.5 andfurther washed. The lignin in this form is known as "A" lignin andcontains 2-3% inorganic salts through additional washing can lower theinorganics to 0.5% or less. This lignin can be reacted with variousorganic or inorganic agents to produce products useful in dyestuffdispersants, asphalt emulsifiers, concrete air entrainers, and otheruseful compounds.

The "A" lignin aggregates which are recovered from the kraft pulpingprocess consist of a popcorn-like, porous structure in which theindividual ultimate particles are fused at their points of surfacecontact to adjacent particles. The bonds between these individualparticles are extremely strong, and attempts in the past to break upthese aggregates by grinding into smaller particles have not beensuccessful in obtaining submicron particles, i.e., particles having amedian diameter of less than one micron (10,000 A). Wet grinding oflignin produces only a particle size as low as about 30 to 40 thousandangstroms diameter.

Early U.S. Pat. No. 3,223,697 discloses a method of producing lignin asa kraft wood-pulping by-product in small particle size by mechanicallycontinuously propelling a stream of an aqueous solution of an alkalinelignin through a mixing zone of constricted cross-sectional area withsufficient velocity to create turbulent flow while continuouslyintroducing a stream of a precipitant for the lignin to mixsubstantially instantaneously with the lignin solution to precipitatethe lignin particles in the mixing zone in small particle sizes.However, such a method requires considerable processing equipment andcareful control of temperatures and pressures at considerable economicexpense, and the random shapes and sizes of the lignin particlesproduced by the method do not allow close packing or particle-particlejuxtaposition necessary for proper film-forming and bindingapplications.

In the use of lignin by-products in the formulation of certain productsand chemical compositions, it is desirable that the lignin particles besmall enough that the particles may form continuous films, particularlyin use of the lignin as a binder or coating in printing, in laminatingpaper, in forming rigid boards, as an adhesive for binding sawdust,woodchips, or plywood, and as a green strength enhancer for ceramicbodies or stain for wood surfaces.

BRIEF OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a ligninproduct of submicron median particle diameter and of a shape and size asto be effectively formed into a film and employed in other chemicalapplications and processes.

It is another object of the present invention to provide an improvedmethod of producing a lignin product of sufficiently small medianparticle diameter as to form films and be particularly suitable forfilm-forming and use as chemical additives in other chemical processesand products, such as adhesives, binders in printing inks, surfacecoating for paper, and the like.

SUMMARY OF THE INVENTION

The present invention is directed to production of lignin materialswhich, in liquid dispersion form, comprise particles having a medianparticle diameter of less than about one micron, or 10,000 angstroms,preferably less than about 300 angstroms, and of such uniform shape andsize so as to be readily employed as film-formers and additives in otherproducts and chemical compositions. The lignin material productcomprises colloidal dispersion of monodispersed lignin particles whichcoalesce and dry to form a continuous film which is particularly usefulas a binder in pigment printing, coating, and composite boardmanufacture.

The method of the present invention comprises the steps of preparing anaqueous solution containing from about 12 to 18% lignin by weight, fromabout 2 to 15 moles ammonia, from about 4 to 12 moles of sodiumhydroxide or potassium hydroxide, per mole of lignin present in thesolution. The solution is heated to at least about 150° F. for asufficient time to cross link the lignin and form a colloidal suspensionof lignin particles of submicron median particle diameter, preferably,less than about 300 angstroms median particle diameter. Solidsconcentration of the colloidal dispersion may be from between about15-20% by weight dry at pH 8.0-10.0. Preferably, the aqueous solutionmay contain about 5 moles ammonia, 6 moles formaldehyde, and 0.75 molessodium hydroxide per mole of lignin.

As the reaction proceeds, the lignin becomes insoluble and precipitatesas small monodispersed particles. No agglomeration of these particlesinto flocs is noted and these precipated particles maintain the originalparticle size until the reaction is complete.

The preferred method of production is illustrated by the followingexample.

EXAMPLE

1. A 750 gallon reactor is charged with 2667 lbs. slurry of "A" lignin(0.8 mole) at 30.0% solids dry weight.

2. The "A" lignin slurry is diluted to a 17.2% solids ligninconcentration with 1995 lbs. of water and agitated 60 minutes.

3. 48 lbs. of 50% solids sodium hydroxide (0.6 mole) is added tocatalyze the reaction.

4. Ammonium hydroxide containing 67.9 lbs. of ammonia (4.0 moles) isadded as agitation is continued for 2 hours.

5. A formaldehyde solution containing 144 lbs. (4.8 moles) of activeformaldehyde is added and agitation continued.

6. The reactants are heated to 185° F. and this temperature ismaintained for 4 hours.

7. The reactants are cooled to 125° F.

8. The final product is a colloidal lignin dispersion of 17.5% drysolids with a median particle diameter of 265 angstroms and a viscosityof 7.0 cps Brookfield at 25° C. pH is 8.66.

To determine median particle diameter size of the lignin productsproduced in accordance with the present invention, a colloidal lignindispersion, as in Example 1, containing 15 to 20% dry solids by weightis diluted with distilled deionized water to a concentration of 300-400parts per million. The diluted sample is placed on an electronmicroscope grid and frozen instantly with liquid nitrogen. The frozengrid is placed in a cold holder and viewed on a Philips 400 electronmicroscope. Photographs are taken at 130,000 magnification.

Utilizing the photographs, the particles are counted and the longestdiameter measured on a Zeiss Mop-3 image analyzer. Particle sizedistribution and median particle diameter is then calculated using thisdata. The term "median" in reference to particle size is used toidentify that particle diameter so chosen in the series that is countedand measured that half the number of individual particles, by diametersize, are above it and other half, by diameter size, are below it.

The products of the present invention can form tightly adhering films ona number of different substrates. When cast on glass, a clear, tantinted film forms which does not craze or turn transluscent on drying. Asimilar result is obtained with an aluminum foil substrate which hasbeen cleaned to remove the processing oils. Cleaned polyethylene canalso be coated, through the adhesion is not as strong as that obtainedon glass. Films are also produced on glossy paper, coated paper,calendered paper, and newsprint.

This film-forming property of the lignin materials of the presentinvention is of great utility in formulating pigmented and dye-coloredinks. In carbon black base ink formulations, a glossy surface isproduced on calendered paper and newsprint. Of particular value is thegood rub resistance and negligible wash off when this product is the inkvehicle. At high carbon black loadings, e.g., above 15%, dry weightviscosity is excellent and stable.

The following examples indicate the utility of this product in inkformulations.

EXAMPLE 2

Three samples of a carbon black composition are prepared, as follows:

1. Water and an Acrysol I-62 pigment dispersant (Rohm & Haas) are mixedat low speed in a 1200 ml Waring blender.

2. pH is adjusted from 3.5 to 8.5 with ammonium hydroxide.

3. Carbon black and the submicron lignin dispersion of Example 1 isadded at moderate speed.

4. After addition the blender speed is increased and agitation continuedfor 15 minutes.

5. The formulation is then shot milled for 30 minutes.

The results were as follows:

    ______________________________________                                        Sample No.     1         2         3                                          ______________________________________                                        Ingredients                                                                   1. CSX 220 Carbon Black                                                                      20%       25%       25%                                        (Cabot Corp.)                                                                 2. Lignin--Example 1                                                                         5%        6%        7%                                         3. Acrysol I-62 (2)                                                                          1%        1%        2%                                         4. Water       74%       68%       66%                                                       100%      100%      100%                                       Results                                                                       pH             8.85      8.8       9.3                                        Viscosity Shell                                                                              S3-19     S3-21.0   S4-21.5                                    Cup--Secs.                                                                    Viscosity CPS  12        28        73                                         Med. Particle Dia.                                                                           0.2 micron                                                                              0.24 micron                                                                             0.24 micron -(Carbon Black &               ______________________________________                                                                           Lignin)                                

Drawdowns of these formulations on calendered paper produced deep glossyblack films. Rub-off was low and wash off was negligible.

These formulations are typical for ink base recipes with high carbonblack loadings and are well suited for water-based flexo graphic inkapplications.

EXAMPLE 3

A lignin dispersion of Example 1 was formulated into an ink for testingon a 55 inch flexo press. The following procedure was used forpreparation.

1. A lignin dispersion of 16.2% solids was diluted to 5% by the additionof 43 lbs of water. This gave 12.5 lbs of dry solids in 68 lbs total.

2. CSX 220 Carbon Black (Cabot Corp.) (12.0 lbs) was mixed with thelignin slurry to give a 15% loading.

3. This intermediate mix was then ground for 1 hour in a pilot scaleshot mill.

4. The ground ink contained median particle diameter of 0.29 microns;total dry solids of 19.4% and pH 9.20. Shell cup viscosity #3 was 32.6seconds or 46.3 cps.

This ink was tested on a 55 inch flexo-graphic printing press. Thematerial was fed to the printing press reservoir and pumped onto theprinting rolls. The paper printed was 30 pound Bears Head, NorthCarolina stock. Initial press speed was 500 feet/minute andincrementally increased to 1100 feet/minute. A good copy was run at thisspeed. The paper was cut and folded. The test was considered to besuccessful and the ink performed well. Rub resistance was consideredexcellent.

Lignin dispersions, the object of this invention, have been found to beeffective in oil base offset printing. The following example illustratesa formulation and the procedure used to prepare this ink.

EXAMPLE 4

Procedure:

1. A lignin dispersion of Example 1 is mixed with Vulkan K carbon black(Cabot Corp.) and REAX 88B sodium lignosulfonate (Westvaco) in a Waringblender for 5 minutes at maximum speed.

2. Bunker C fuel oil and linseed oil are added and agitation continuedfor 5 minutes.

3. Magie Oil 55 (ink oil--Magie Corp.) is added and the agitationcontinued for an additional 5 minutes.

4. The ink is then milled in a shot mill for one hour.

Formulation:

    ______________________________________                                        1. Bunker C -                                                                            32.6%                                                              Residual fuel oil                                                             2. Magie Oil 55 -                                                                        28.8%                                                              Ink oil - Magie Co.                                                           3. Linseed Oil -                                                                         2.6%                                                               Drying oil                                                                    4. Vulkan K -                                                                            10.4%                                                              Carbon black - Cabot Corp.                                                    5. Lignin -                                                                              3.5%                                                               Lignin dispersion                                                             6. REAX 88B -                                                                            0.8%                                                               Sodium lignosulfonate - Westvaco                                              Corp.                                                                         7. Water - 21.3%                                                                         100.0%                                                             ______________________________________                                    

The final ink was stringy, water compatible and passed the grind gaugetest with no particles above 2.5 microns. Drawdowns on calendered papergave a lustrous deposit with good color development. Rub was low andwater wash negligible.

In order to get inks with deeper black shades ink manufacturers havegone to higher carbon black loadings. This approach gives inks of higherthan normal viscosity with rheological flow problems on high speedpresses. In addition, it is felt that high carbon black loadingsincrease "fill in" in printing. This gives fuzzy reproduction andaffects contrast. Considerable interest exists in flexographic printingfor systems which give deep black shades without high carbon blackloadings. One approach is to replace part of the black particles withblack dye and still retain the desired shade. This has been difficult toachieve because the dye must be insolubilized and adhere to the inkvehicle.

The product of this invention has been found to effectively fix the dyeand lower the carbon black loading in the final ink. This is shown inthe following example and data.

EXAMPLE 5

Procedure:

1. A lignin dispension of Example 1 is placed in a 1200 cc Waringblender and diluted with water.

2. After mixing the slurry is heated to 165°-175° F.

3. A black disperse dye is added at high speed agitation for 3-5minutes.

4. Carbon black is added and the contents mixed for 5-10 minutes at highspeed.

Results:

    ______________________________________                                        Lignin Dispersion, Carbon Black and Sodyeco Black 4GCF                             Lignin    Vulkan   Black                                                      Disersion K        4GCF   Water                                          Run  % (wgt)   % (wgt)  % (wgt)                                                                              % (wgt)                                                                              Sequence of                             No.  A         B        C      D      Addition                                ______________________________________                                        1    5.7       4        3.2    87.1   A+D+C+B                                 2    5         5        3      86.6   A+D+C+B                                 3    6         5        4      85     A+D+C+B                                 4    5         5        4      86     A+D+C+B                                 5    6         6        3      85     A+D+C+B                                 6    6         5        4      84.4   A+D+C+B                                 7    6         6        3      85     A+D+C+B                                 8    5         5        4      86     A+D+C+B                                 9    6         6        4      84     A+D+C+B                                 10   6         6        4      84     A+D+C+B                                 11   6         5        4      85     A+D+C+B                                 12   6         5        4      85     A+D+C+B                                 13   10        6        4      80     A+D+C+B                                 14   5         15       --     80.0   A+D+B                                   ______________________________________                                         1. Lignin Dispersion  Westvaco Corp.                                          2. Vulkan K  Black ink pigment  Cabot Corp.                                   3. Black 4GCF  Soluble, prereduced blackdye  Sandoz                           4. Tamol SN  Dispersant  Rohm & Haas                                          5. Hydrogen Peroxide  Reagent  Baker Chemical                                 6. Potassium Hydroxide  Reagent  Baker Chemical                          

These formulations were deposited by making drawdowns on calenderedpaper and newsprint. After drying, they were tested for water wash andrub. Almost without exception, water wash was minimal and rub wasnegligible. The absence of water wash indicates the fixing of the watersoluble dye in an insoluble coating on the lignin particles.

EXAMPLE 6

Lignin dispersions, the subject of this invention, have been found tofunction as an excellent suspending agent and binder for coating clays.The coatings adhere strongly to kraft paper and form a smooth film whichseals the surface of the paper. These coatings also accept water andsolvent base inks for printing.

The application of lignin dispersion-clay coatings was tested on kraftpaper by handroll proofing and rod coating draw downs. Once dried, thecoated paper was tested on a Teledyne Gurley Porisimeter, Model 4110.The results are shown below.

    ______________________________________                                                                       Uncoated                                                                             Coated                                       Paper                     Control                                                                              Control                                 Run  Used       Formulation    Secs   Secs                                    ______________________________________                                        1    83lb S3    Hydroprint 45% 45.5   166.2                                   2               Lignin Dispersion 5%                                                                         45.5   173.3                                   3    83lb S4    Hydroprint 36% 38.8   119.1                                                   Lignin Dispersion 5%                                                          TiO.sub.2 9%                                                  *4   83lb S3 Mod                                                                              Hydroprint 36% 38.8   252.3                                                   Lignin Dispersion 5%                                                          TiO.sub.2 9%                                                  ______________________________________                                         *Run 4  Rod coated instead of hand proofed                               

These coatings exhibited no rub-off or chalking and no water wash. Whenthese coated papers were pressed at 290° F., 500 psig for 5 minutes, thefilms became glossy and tougher. These properties indicate the value ofthese formulations for improving appearance and printability of kraftpaper for box applications.

Another use for lignin dispersion where it improves water resistance isin laminating kraft liner board or saturating grade kraft paper.

EXAMPLE 7

Two pieces of Westvaco 43 lb. liner board are laminatied by sandwichinga lignin dispersion of Example 1 on the inner surfaces. The dispersionsolids comprise 2.5% by weight of the two sheets. The sheets arelaminated by pressing at 500 psig and 295° F. for a period of 4 minutes.A tight, unpeelable bond is developed. The laminate is boiled indistilled water for 2 hours without any evidence of separation.

EXAMPLE 8

The lignin dispersions are effective as wood adhesives creating a strongwater resistant bond. They may also function as extenders for otheradhesives in this application. The following example illustrates the usewith wood.

Two pieces of outdoor plywood 3"×5"×1/2" are coated on the inside with alignin dispersion of Example 1. The coated sides are placed together andthe block placed in a heated press. The piece is kept at 300° F. and 150psig pressure for 10 minutes, then removed and cooled. The test piece isimmersed in boiling water for one hour, and shows no separation. Anothertest piece kept in water for 7 days also did not separate.

That which is claimed is:
 1. A carbon black composition comprised ofover 15% carbon black, on a dry weight basis, a dispersant, water, and acolloidal lignin dispersion wherein the lignin is "A" lignin derivedfrom the kraft pulping process and the dispersion is characterized bymonodispersed lignin particles exhibiting a median lignin particlediameter size of less than about 300 angstroms.
 2. The carbon blackcomposition of claim 1 wherein the carbon black loading is 25%.
 3. Thecarbon black composition of claim 1 further comprising black dispersedye.